Adaptive equalizers are used in modem electrical communication to equalize (i.e., compensate for) linear amplitude and phase distortions which naturally tend to occur within the useful frequency band of practical transmission circuits. If these distortions are compensated, as is well known in the art, a given circuit is capable of correctly receiving transmission of information at substantially higher rates and over virtually unlimited transmission distances. In particular, the current invention relates to dynamically adaptive equalizers, which employ the signals transmitted through a communication circuit or channel for measurement of its distortion. Such circuits alter their amplitude and phase characteristics with time to minimize the measured error at the receiving end of the circuit. Adaptive equalizers are required for use in connection with circuits the distortion properties of which are unknown at the time transmission is initiated, or which for any reason may change during transmission. Representative circuits include those having variable multiple paths, radio transmissions carrying digital voice, data and other signals, and switched telephone lines. In typical adaptive equalizer operation, communication may be established using a repeated signal carrying no information. The equalizer adjusts rapidly to compensate for the transmission circuit distortion. After information transmission is begun, the information signals are then used continuously for iterative dynamic correction of the filter characteristics.
Digitally operating adaptive equalizers are well known in the electrical communications art. All equalizers are, basically considered, electrical filters; and adaptive equalizers are based on electrically adaptable filters, typically taking the form of finite impulse response filters, some forms of which are described by Watanabe in U.S. Pat. No. 4,771,395 issued Sep. 13, 1988 and entitled FIR Digital Filter.
In addition to an dynamically adaptable electrical filter, an adaptive equalizer requires a means to produce the series of control inputs (termed "weights") which define the filtered output in time when responding to a single input pulse of known amplitude. This means is conventionally referred to as a "weight generator", and its outputs as "weights", though they are actually measures of the response of the filter to a pulse input supplied at evenly spaced time intervals.
The weight generator, in turn, uses as its input an error signal which is derived by comparing the output of the filter with expected value(s) of the output. Since these circuits are used in digital data transmission, the desired output signals have a very limited number of values.
Some prior art adaptive equalizers use identical circuits for the FIR filter and weight generator (WG) functions. An example of such adaptive equalizer is found in the Mobile Link 1/2 Receiver Program (MLRP) receiver, a product of Stanford Telecommunications, Inc., of Sunnyvale Calif., which uses the Zoran 891 FIR filter chip for both the FIR and weight generator functions of its adaptive equalizer. The Zoran chip is second-sourced by Harris as the HSP43891 and employs the canonical structure, which utilizes output weighting.
Currivan and Ohlson in U.S. Pat. No. 5,416,799 issued May 16, 1995 and entitled Dynamically Adaptive Equalizer System and Method describe the use of the inverse canonical structure, which employs input weighting, for the FIR filter and weight generator functions. A stated objective of their invention is to provide an adaptive digital filter (equalizer) the design of which makes use of similar inverse canonical structure circuits for the digital elements of the greatest complexity, i.e. the FIR filter and the weight generator. The practical benefits are reduced design time and reduced production cost for the equalizer. These benefits apply to adaptive filters having both filter and weight generator on a single semiconductor chip, as well as to adaptive filters in which filter and weight generator comprise separate chips or collections of chips and other components. Another stated objective of their invention is to minimize the number of additional integrated circuits required ("glue chips") to construct a family of adaptive equalizers from a multiplicity of integrated circuits of the same design. Another objective of their invention is to employ low-cost integrated digital circuits capable of operation at significantly higher speeds than are required to handle information signals, and through multiplex use of their outputs achieve significant reduction in cost and complexity of an adaptive filter for those information signals.
To secure their objectives Currivan and Ohlson rely on a weight generator that in effect reverses the direction of time for data samples. As a result, the updating of filter weights in an FIR filter with inverse canonical structure is done by Currivan and Ohlson in such order that error signals are generated from kernels that are only partially updated.
Laud in U.S. Pat. No. 5,392,315 issued Feb. 21, 1995 and entitled FIR Filter Coefficient Updating System points out that it is desirable to update the filter weights in an FIR filter with inverse canonical structure in such order that error signals are generated from kernels that are completely updated. To implement such updating of filter weights, a weight generator is required which operates on principles different from those known to the prior art.